It's a slug.

i blanketed my raised bed with some Garden Safe Slug and snail bait, but they're still around... would BT help?

No, BT is for caterpillars. You could take sardine tins (or other shallow tins) and fill them with beer. The slugs crawl in and drown.......

Slug/snail bait does’t kill them immediately, but as soon as they eat it they will stop feeding on your plants.

That looks like a baby slug to me. I can also attest to the effectiveness of iron phosphate bait for slugs and snails, but you have to keep at it, since rain and water can wash it away, and it does get eaten.

I make a circle with the pellets around the base of plants I want to protect, and make sure the plant isn't touching anything that could provide the pests with a path that avoids the bait.